Today's digital mobile radio-communications systems are generally set up as cellular networks. A given access point, referred to below as the base station (BS), forms a given radio cell. In this case, the size of the cell is determined by the field propagation, the desired transmission rate and the number of mobile stations (MS) in radio contact with the base station. In current systems, these cells are subject to significant size constraints, as high frequencies are used and there are large numbers of mobile stations; as a result, the way the mobile station is passed from one cell to a neighboring cell is of major significance. Passing the mobile station link from radio cell 1 to radio cell 2 is termed ‘handover’ (HO). In this context, there are basically two kinds:                Handover in which active links are cleared (e.g., DECT)        Handover in which active links are handed off to the new BASE STATION (e.g., GSM).        
There are various sub-categories of handover:
Soft handover: Handing off without the loss of individual data.
Hard handover: Handing off in which individual data are lost.
Forward handover: The mobile station seeks its target base station itself and registers there directly.
Backward handover: The mobile station registers HO at the old base station, which then performs the search for a suitable new base station.
Non-network supported: (forward HO only) When registering at the new base station, the mobile station itself is responsible for ensuring that all link parameter settings and link settings are modified.
Network supported: The network ensures that the two base stations involved can communicate directly. In this case, the old base station sends the new base station all relevant information regarding the mobile station and its links.
Mobile-station-initiated handover: The mobile station determines that an HO is required and initiates this process.
Base-station-initiated handover (forced HO): The base station wants to free up capacity or it determines that the radio link is deteriorating and informs the MS that it is to carry out an HO.
Currently, handover methods, e.g., in GSM, require that the backbone network be able to support the handover. However, in the future this cannot be taken for granted, as a number of different backbone networks may be connected. As a result, there are certain constraints regarding certain special combinations of HO types.